Ofloxacin ((.+-.)-9-fluoro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-2,3-dihydro-7H -pyrido[1,2,3-de][1,4]-benzoxazine-6-carboxylic acid) is known to be an excellent synthetic antimicrobial agent as disclosed in Japanese Patent Application (OPI) No. 46986/82 (the term "OPI" used herein means "unexamined published patent application").
Ofloxacin has an asymmetric carbon atom at the 3-position thereof and is obtained as a racemate (specific rotation [.alpha.].sub.D =0.degree.) by known processes. The present inventors obtained optically active compounds of the racemic Ofloxacin and found that the S(-)-compound possesses an antimicrobial activity of about 2 times higher than that of the (.+-.)-compound and an acute toxicity (LD.sub.50) weaker than that of the (=)-compound as determined in mice by intravenous administration. On the other hand, the present inventors have found that the R(+)-compound exhibits an antimicrobial activity of only about 1/10 to 1/100 times that of the (.+-.)-compound, whereas it possesses an acute toxicity substantially equal to that of the (.+-.)-compound. That is, the S(-)-form of Ofloxacin has been found to have very desirable properties, i.e., increased antimicrobial activity and reduced toxicity, and is expected to be a very useful pharmaceutical agents as compared with the (.+-.)-compound. Further, both the (R+)- and S(-)-compounds of Ofloxacin in the free form have markedly high water-solubility as compared with (.+-.)-compound and as compared with free compounds of this type, and can be used as injectable preparations. These advantages will be apparent from the experimental data shown hereinafter.